1. Field of the Invention
The invention relates to a process for preparing organopolysiloxane compositions in a kneading cascade having at least two kneading chambers arranged in series next to one another, followed by a reciprocating kneader.
2. Background Art
EP 570387 A describes a process for the continuous preparation of HTV silicone compositions, in which the incorporation of filler into the polysiloxane and degassing are carried out in one step in a single-screw reciprocating kneader. However, the machine can also be segregated so that the mixing process can take place in the first kneader and the degassing step can take place in the second kneader. Owing to phase inversion, dry friction, and the mechanical destruction of high molecular weight polyorganosiloxanes, the storage stability of the resulting silicone composition is reduced. In addition, satisfactory throughputs cannot be achieved using this type of apparatus.
A further process development is described in EP 1033389 A. To improve storage stability in particular, the process is extended by rubber recirculation which is supposed to prevent phase inversion and thus to reduce the tendency of the HTV compositions prepared in this way to become stiff and brittle. Two-stage processes are not mentioned in EP 1033389 A. The process is complicated in an engineering sense by the recirculation of product, so that economical operation is not possible. In this process, the throughput is also in need of improvement.
EP 1110696 A describes a two-stage process for preparing HTV silicone compositions. In the first stage of this process, mixing of filler, silicone polymer, hydrophobicizing agents, and plasticizers takes place in a co-rotating twin-screw extruder. In the second stage of the process, the mixture is then compounded fully and degassed in a single-screw reciprocating kneader. Compared to EP 570387 A, the process of EP 1110696 A is alleged to be superior in terms of throughput and improved filler distribution. However, EP 570387 A states that twin-screw machines are problematic in terms of machine wear and abrasion of metal and can result in undesirable gray coloration of the HTV compositions. This applies particularly to highly filled, stiff mixtures.
EP 807509 A describes the continuous preparation of storage-stable organopolysiloxane compositions using a specific cascade of kneading machines to provide base compositions for addition-crosslinking liquid rubbers. Degassing of the composition occurs only via a degassing vessel, not by means of a specifically designed machine. According to EP 807509 A, HTV compositions can also be prepared using a kneading cascade. Owing to the lack of opportunities for degassing or unsuitable degassing facilities, HTV rubbers produced by this process are unusable.